1. Field of the Invention
This invention relates to a method and apparatus for the recovery of heat from a hot gas flow. More particularly, this invention relates to a process for the recovery of heat from a flow of hot product gas produced in a coal gasification plant wherein the recovered heat is used to produce a flow of superheated steam which may be used in the gasification plant.
2. Description of the Prior Art
Copending U.S. patent application of P. G. Garside, Ser. No. 264,479 filed May 18, 1981, discloses a process for the production of a combustible product gas from coal in a rotary kiln gasifier. As disclosed in the aforesaid application, the gasifier generates two flows of a product gas. A first flow consists of product gas laden with vaporized tars and particulate matter. A second flow consists of product gas free of vaporized tars and containing only particulate material. The aforesaid application discloses the second flow of product gas, which is hotter than the first flow, is regulated to maintain the temperature of the first flow sufficiently high to prevent condensation of the tars within the first flow. Accordingly, in a coal gasification process as described, the second flow containing tar-free gas may, at times, be completely diverted to maintain the desired temperature of the first flow. At other times, when the entire second flow is not diverted, the remainder of the second flow is processed to remove the particulate matter.
In a process as described in the Garside application, it is economically advantageous to recover the sensible heat from the second flow after particulate removal. For example, the recovered heat can be used to produce a source of superheated steam which may be used in the gasification process as described in the aforesaid application or used in a steam turbine to generate electricity in a combined-cycle power plant.
In the prior art, heat exchanger systems for utilizing the heat of hot gases to produce steam are well known. Such conventional systems typically pass the hot gas through a superheater for indirectly heating a flow of saturated steam to convert the saturated steam to superheated steam. Saturated steam is steam which, for a given pressure, is at a temperature such that any reduction in the temperature results in water condensing within the steam. Superheated steam is steam which, for a given pressure, is at a temperature such that the temperature of the steam may be reduced a substantial amount before any water condenses.
In the conventional heat exchanger system, the temperature of the hot gas is reduced by reason of the heat transfer within the superheater. The reduced temperature gas is passed to a boiler for heating saturated water to generate saturated steam at a prescribed pressure and yet further reduce the temperature of the gas. The saturated steam produced in the boiler becomes the source of saturated steam for the superheater. Saturated water is water which, for a given pressure, is at a temperature (called the saturation temperature) such that any additional transfer of heat to the water will convert the water to steam rather than further increasing the temperature of the water. Unsaturated water is water which, for a given pressure, is at a temperature less than the saturation temperature.
Conventional steam generating systems, as described above, present significant problems in applications for heat recovery in gasification processes such as disclosed in the aforesaid application of P. G. Garside. As mentioned, the tar-free second gas flow, from which sensible heat is to be recovered, may be flowing intermittently. As a result of the intermittent nature of the flow, the superheater may, at times when no gases are passing through the system, become empty and dry. Subsequently, a flow of hot product gas (which may be of a temperature in excess of 1,800.degree. F.) is admitted to the dry superheater. The sudden introduction of hot gas to the dry superheater presents a potentially dangerous situation since the temperature differential between the gas and the dry superheater creates a thermal shock which may damage the superheater. For superheaters fabricated from conventional material, such as carbon or stainless steel, a gas temperature in excess of a safety temperature, such as 1,100.degree. F. to 1,300.degree. F., presents a substantial risk of damage due to thermal shock when the gas is introduced to the dry superheater.
Two solutions have been suggested to resolve the above-mentioned problem. The first is to fabricate the superheater of special alloys, such as Incoloy and Inconel alloys, which can withstand the extreme thermal shock. However, such alloys are very expensive and such an approach is not an economical alternative for many applications.
A second solution is to provide the heat recovery system with an auxiliary boiler with a conventional furnace for producing saturated steam which is fed to the superheater prior to introducing the product gas to the superheater. Again, such a solution is very expensive since it requires auxiliary equipment.